It is known that when a halomagnesium salt of a tetrakis(aryl)borate anion is obtained, reacting the obtained halomagnesium salt with sodium carbonate or sodium bicarbonate yields the corresponding sodium salt of the tetrakis(aryl)borate anion; see Nishida et al., Bull. Chem. Soc. Jpn., 1984, 57, 2600, Fujiki et al., J. Fluorine Chemistry, 1992, 57, 307, and EP 913,400. However, the yields of the sodium salt are not always satisfactory; further, Goldberg et al., in Zhurnal Organicheskoi Khimii, 1989, 25, 1099, state that the products obtained by Nishida et al. are hydrates. The presence of water in the alkali metal salt of the tetrakis(aryl)borate anion is often undesirable; thus, a process which produces high yields of an alkali metal salt, and especially a dry alkali metal salt, of the tetrakis(aryl)borate anion from the halomagnesium salt is needed.